1. Field of the Art
The present invention relates to a cleaning water production system and, more particularly, to a system for producing cleaning water used in the production of semiconductors, chemicals, foods, etc., particularly in the production of megabit-class semiconductors.
2. Prior Art
A large amount of high-purity cleaning water is used in the production of semiconductors, chemicals, foods, etc. Particularly, in the production of semiconductors, water of an extremely high degree of purity is used for cleaning in photoetching and other processes. In this case, cleaning water must have an extremely high degree of purity, generally called "ultrapure water", so as not to contaminate the semiconductors.
Cleaning water used in a conventional semiconductor manufacturing process has heretofore been produced as follows.
As shown in FIG. 5, city water 1 is treated in a primary pure water production system 2 to produce pure water 3, and the pure water 3 is further purified in a subsystem 4 to produce ultrapure water 5, which is supplied to each point of use 12p, 12q, 12r, 16p, 16q, 16r, 17p, 17q, 17r through a supply main pipe 6, supply subpipes 7a, 7b, 7c and supply branch pipes 13p, 13q, 13r . . . In this ultrapure water production system, the amount of ultrapure water produced in the subsystem 4, that is, the feed rate of ultrapure water, is constant at all times, and ultrapure water remaining unused at the points of use is returned to the subsystem 4 through return branch pipes 13p, 13q, 13r . . ., return subpipes 14a, 14b, 14c and a return main pipe 15 as return water. The reason for this is that the quality of ultrapure water is degraded even by a slight amount of impurities, which may liquate from the supply pipes even when the supply pipes are produced from a high-purity material. Therefore, if the supplied ultrapure water 5 is allowed to reside in the supply pipes when it is not used at the points of use as in the case of ordinary tap water, the purity of the ultrapure water is immediately lowered due to impurities liquating from the supply pipes, resulting in water unfit for use as ultrapure water. For this reason, it is necessary to keep ultrapure water flowing through the supply pipes at all times regardless of use. Pure water having a lowered level of purity due to its flowing through the return pipes returns to the subsystem 4 where it is repurified before being supplied to the supply pipes.
When city water is used, the above-described primary pure water production system 2 is generally constructed of an activated carbon filter, a deaerator, a reverse osmosis membrane, and an ion exchanger. When lake water or river water is used, the primary pure water production system also employs a coagulo-setting filter.
In general, the above-described subsystem 4 is a system in which pure water treated in the primary pure water production system is further treated to produce water of a high degree of purity, that is, ultrapure water, by using an ultraviolet sterilizer, an ion exchanger, and an ultrafiltration membrane.
Incidentally, in the semiconductor manufacturing field, as technology advances, the integration of LSIs has become increasingly high, i.e., from 16-megabit LSIs to 64-megabit LSIs and further to 256-megabit LSIs, and concurrently cleaning water having a higher degree of purity has been demanded. Accordingly, the process for producing such LSIs requires the use of cleaning water having an extremely low content of dissolved ions and oxygen and containing an extremely small number of solid particles and bacteria. Thus, cleaning water used in the semiconductor manufacturing process must be extra-ultrapure water, which is higher in purity than the conventionally used ultrapure water.
As a result of the advancement of the technique for producing ultrapure water, an extra-ultrapure water production system capable of supplying extra-ultrapure water that is higher in purity than ultrapure water has been developed. However, the purity of extra-ultrapure water is lowered due to impurities liquating from supply pipes during its flow therethrough, as described above.
The above-described problems could be eliminated by supplying extra-ultrapure water without using supply pipes or making the length of the supply pipes extremely short. That is, it may be a solution to the problems to provide an extra-ultrapure water production system capable of obtaining extra-ultrapure water from city water at each point of use. However, in a semiconductor manufacturing factory, where human contact should be avoided as much as possible, it is impossible to provide an extra-ultrapure water production system at each point of use from the viewpoint of the arrangement and management of the system.